The present invention relates to an image forming apparatus including: an image carrier unit integrally having a plurality of image carriers, which respectively carries an image; and a unit containing portion which removably contains the image carrier unit. The present invention also relates to a technique associated with the image forming apparatus.
Some type of a conventional laser printer includes a photoreceptor unit, having a plurality of photoreceptor drums integrally aligned in one direction, and a unit containing portion, wherein the photoreceptor unit can be removably contained along the alignment direction of the photoreceptor drums.
In this type of laser printer, drum gears are provided and respectively connected to the photoreceptor drums so as to transmit driving force to the photoreceptor drums for rotating the drums. Moreover, a plurality of driving gears is provided in the unit containing portion. The driving gears are engaged with the respective drum gears so as to transmit driving force to the drum gears from a motor.
Generally, in this type of laser printer, when the photoreceptor unit is removed while a user lifting an end portion of the photoreceptor unit in a downstream side of the removal direction, the drum gears and the driving gears are disengaged in a consecutive manner from the downstream side.
The drum gears and the driving gears are formed in such a manner that the center of each circle works as the center of rotation. However, in a precise sense, the center of rotation slightly deviates from the center of the circles. Therefore, the displacement rate on the outer circumference of the gears at the time of rotation (the displacement amount on the outer circumference per unit time) is not constant.
When images, carried by respective the photoreceptor drums, are sequentially superposed on a sheet of paper conveyed in the above-described laser printer, the respective images are misaligned due to the dislocation of the center of gear rotation. A solution is required so as to inhibit misalignment of images carried by the photoreceptor drums.
For this purpose, in the above-described laser printer, a phase reference point is predetermined for the respective gears based on the dislocation of the rotational center, and the phase differences between the adjacent drum gears and between the adjacent driving gears are set so as to be respectively constant.